1. Technical Field
The present technology pertains generally to gas production systems and methods, and more particularly to an improved hydrogen gas steam reformation system that is energy and cost efficient and does not require burning of fossil fuels for heat.
2. Background Discussion
Worldwide population growth and industrial expansion has produced a dramatic increase in the consumption of oil and other fossil fuels. Global emissions from fossil fuels have had a substantial impact on the environment. Hydrogen is a promising alternative to the use of fossil fuels that may mitigate the deleterious effects of burning hydrocarbons and limited fossil fuel supplies.
Hydrogen fuel can react with oxygen to release energy in engines to produce water rather than greenhouse gases. Fuel cells are a developed technology that directly converts the chemical energy of the hydrogen fuel into electricity and heat without involving combustion. A fuel cell is an electrochemical device with an anode and a cathode separated by a thin layer of electrolyte. Typically, hydrogen reacts in a fuel cell on the anode side and oxygen gas or air reacts on the cathode side. Fuel cells can produce electrical energy continuously as long as fuel and an oxidant are provided to the electrodes.
Compared to conventional combustion methods of producing electricity, hydrogen based fuel cells are considered to be attractive alternative because of zero-emissions and high efficiency. Unfortunately, hydrogen fuel for fuel cells is not naturally occurring as a collectable gas and it must be generated from a secondary source. Approximately 95% of the hydrogen produced today comes from carbon based raw materials, such as methane or natural gas. However, it is difficult to use fuel cells in many applications due to the lack of ready available hydrogen, storage and distribution infrastructure.
The overall profitability of fuel cells for producing energy is nearly double that of a conventional fossil fuel combustion engine making them attractive producers even if the hydrogen is obtained from methane or other fossil fuels. In addition to the production of electricity, hydrogen can be used as a chemical feedstock for petrochemical, food, electronics and metallurgical processing industries. For example, hydrogen can be used in refineries as raw material for the hydrocracking of oil associated with gasoline production. Hydrogen is also envisioned to be an energy carrier for vehicular transportation through its use in hydrogen fuel-cell-powered cars.
In the short term, increased production of hydrogen gas will be with the use of conventional technologies, such as natural gas reforming. Catalytic steam reformation of methane is a well-known, commercially available process for the production of hydrogen. Typical hydrogen production is accomplished through several steps: steam reforming, water gas shift reaction, and hydrogen purification. However, carbon is converted to CO2 that is ultimately released to the atmosphere with these processes and therefore alternatives to the atmospheric release of CO2 must be created.
Steam reforming of natural gas and other light hydrocarbons is currently the most economical process for hydrogen production. The most common method of producing commercial bulk hydrogen is methane steam reforming (MSR), that has an overall reaction of CH4+2H2O═CO2+4H2. The steam reforming process is typically carried out industrially at around 1100K and the process is highly endothermic. The initial steam reforming reaction of CH4+H2OCO+3H2Δh=+206.16 kJmol−1 CH4 is endothermic and requires an input of external heat. Combustion of a portion the natural gas feedstock and waste gases from the hydrogen purification system are often used to produce the external heat load that is required in order to drive the reaction.
After reforming, the resulting gasses are sent to one or more shift reactors, where the hydrogen output is increased with the use of the water-gas shift reaction CO+H2O═CO2+H2ΔH2=−41.15 kJmol−1 CO that is exothermic. Typically, this shift reaction can take place in two stages and reactors. The first stage takes place in a high temperature shift reactor followed by a second stage in a low temperature shift reactor. The gases emerging from the shift reactors contain predominantly hydrogen with small quantities of H2O, CO2, CH4 and CO. The hydrogen is then purified from the reactor gases by available purification schemes. For example, pressure swing absorption (PSA) systems or catalysts like palladium membranes can be used to purify the hydrogen.
It can be seen that the high endothermic heat load makes methane steam reformation a capital and operating cost intensive process. A reduction or elimination of the high endothermic heat load of the process would improve the economics of this process considerably. To this end, a commonly employed method is autothermal reforming, where oxygen is injected into the reforming mixture, leading to significant reduction, or even possible elimination of the aforementioned heat load. This approach, however, is not widely practiced due to its inherent safety concerns (oxygen/hydrogen mixtures have a wide explosive range) and the need for an air separation subsystem which will provide the oxygen needed for the autothermal reforming process.
Accordingly, there is a need for systems and methods for hydrogen production that has a reduced heat load so that the associated system costs are greatly reduced. In addition, there is a need to incorporate renewable energy resources to possibly supply either all or part of this heat load. The present technology satisfies these needs and is generally an improvement in the art.